Which joints are involved in scapulohumeral rhythm?

• A. Glenohumeral joint and scapulothoracic articulation
• B. Scapulothoracic articulation and acromioclavicular joint
• C. Glenohumeral joint and acromioclavicular joint
• D. Glenohumeral joint and sternoclavicular joint

Answer: (C)

Scapulohumeral rhythm describes how the motion of the arm at the shoulder joint is coordinated with the movement of the shoulder blade as it lies on the thorax. When you lift your arm, the humerus mainly drives the activity at the glenohumeral joint, but the scapula must rotate and tilt to keep the glenoid facing the humeral head. That scapular adjustment is made possible by joints that connect the scapula to the rest of the skeleton, especially the acromioclavicular joint where the clavicle meets the acromion. The AC joint allows the scapula to rotate and tilt in step with arm elevation, enabling smooth upward rotation of the scapula as the arm rises. So, the two joints most involved in enabling this coordinated movement are the glenohumeral joint and the acromioclavicular joint, with the scapula’s motion on the thorax (scapulothoracic relationship) accommodating that rotation. The combination supports the typical rhythm in which humeral elevation and scapular movement proceed in a coordinated, about 2:1 ratio.